Vaisala GMP231 - Manual (Page 77)

Chapter 6 _______________________________________________________________ Maintenance
VAISALA ________________________________________________________________________ 75
- The effect of background gas compensations for humidity and oxygen
may be significant when using calibration gases, since they are often
dry and oxygen-free. For example, pure nitrogen gas is typically used
as a convenient 0 ppm CO
2
reference.
- Remember to restore the normal compensation settings after
completing calibration and adjustment. If you are integrating the
calibration functionality of the GMP231 as part of the incubator’s
control software, also implement proper handling of the
environmental compensations.
Limits of Adjustment
GMP231 limits the amount of adjustment that is allowed to the CO
2
measurement. The maximum correction that you can apply is
1000 ppm + 25 % of the probe’s uncorrected reading. Previous user
adjustments do not affect this limit (correction is not cumulative). This
feature limits the possible error introduced by incorrect adjustment.
For example, if you are adjusting using a 5 %CO
2
calibration gas
(50000 ppm), the maximum correction you can apply is approximately
13500 ppm. Attempting to apply a greater adjustment will fail.
Notification of failure from the GMP231 depends on the interface you
are using for adjustment.
Adjustment Types
You can adjust the CO
2
measurement of the GMP231 in one or two
points.
- One-point adjustment is recommended if you are interested in
maintaining a fixed CO
2
level inside the chamber. For best result, use
a calibration gas with a CO
2
concentration that is close to the intended
level.
- Two-point adjustment is recommended if you typically measure a
variable CO
2
level.
Available adjustment functions depend on the interface you use to
operate the GMP231. If you want to integrate the functionality into the
incubator’s control system, the I
2
C interface and the Vaisala industrial
protocol are recommended. If you want to compare the reading of the
GMP231 to a reference instrument as adjust it accordingly, use an MI70
hand-held indicator and a reference probe.